IP Multimedia Core Network Subsystem (IMS) is an IP-based network architecture proposed by the 3rd Generation Partnership Project (3GPP), and it constructs an open and flexible service environment, supports multimedia application, and can provide plentiful multimedia services for the subscribers.
In an IMS service system, the control layer is separated from the service layer. The control layer does not provide specific services, and only provides necessary functions such as trigger, routing and charging for the service layer.
The service triggering and control function in the control layer is implemented by a Call Session Control Function (CSCF). The CSCF is divided into three types: Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF), and Serving-CSCF (S-CSCF), wherein, the S-CSCF takes the main responsibilities, and the I-CSCF is optional.
The service layer is composed of a series of Application Servers (ASs), and can provide specific services. The AS may be an independent entity, or may be present in the S-CSCF.
The control layer (S-CSCF) controls service triggering according to the subscription information of the subscriber, revokes the services on the AS, and implements the service functions.
The end-to-end equipment in a session is called a User Equipment (UE), which is responsible for interaction with the user. Some UE has multiple modes for accessing the network, including accessing the network through a 3GPP Packet Switch (PS) domain, through other non-3GPP PS domains, or even through a Circuit Switch (CS) domain, etc. The IMS network also has an Interconnect Border Control Function (IBCF), which is used for interaction between IMS networks of different operators.
If the CS network is configured with an enhanced Mobile Switch Center (eMSC), and an SIP (Session Initiation Protocol) interface is provided by the eMSC for the interaction with the IMS network, then the interaction between the IMS network and the CS network can be implemented by the eMSC.
For the UE with multiple access modes, if the UE can only use one of the modes at a certain moment and it is implementing a call service under the access mode of 2/3G network, since the call service is provided by the CS domain in the 2/3G network, when the UE moves to other places and thus needs to change the used access mode to accessing through a LTE (Long Term Evolution) network or HSPA (High Speed Packet Access) network, the UE and the network can provide a certain mode to ensure that the call service being implemented by the UE is not interrupted. Since the call service is provided by the PS domain in these networks, such an ability is called as reverse Single Radio Voice Call terminal Continuity, which is in short reverse Single Radio Voice Call Continuity—rSRVCC. Correspondingly, if the UE moves from a LTE network or HSPA network to a 2/3G network, it is forward Single Radio Voice Call terminal Continuity, which is in short Single Radio Voice Call Continuity.
There should be two processes for implementing the rSRVCC: one is a handover process from the original network to the destination network, and the other is a media connection establishment process between the terminal and the far end. Currently, the handover process already has a clear implementation flow (see FIG. 1) in the industry, but a clear solution for how to initiate the media connection establishment process between the terminal and the far end is still to be proposed.
FIG. 1 is the existing flowchart for rSRVCC, which describes that a call is established between UE-1 and UE-2, and UE-1 establishes the call through the CS domain of a 2/3G network, so what is connected with UE-1 in the call media path is a segment of CS media connection, and after UE-1 performs rSRVCC, the process of UE-1 handing over from the CS domain to the PS domain includes the following steps:
in step 101, UE-1 sends a measurement report to the original network serving the UE-1 to report the cell signal strength measurement information;
in step 102, the base station system of the original network serving the UE-1 judges according to the signal strength measurement information of each cell in the measurement report that a near PS destination network is mostly suitable for serving UE-1, and then decides to perform a handover operation, and the base station system of the original network sends a handover request, for example a Handover required message, to the MSC;
in step 103, the MSC sends a handover request, for example a CS to PS Handover Request message, to a destination network control entity, and the control entity may be a Mobility Management Entity (MME) of the LTE network or a Serving GPRS Support Node (SGSN) of the HSPA network;
in step 104, if the UE-1 also has data services, the base station system of the original network will also send a handover request, for example a Relocation Required message, to the data domain control entity (i.e., SGSN) of the original network;
in step 105, after step 104, the data domain control entity of the original network sends a handover request, for example a Forward Relocation Request message, to a destination network control entity;
in step 106, the destination network control entity sends a handover request, for example, a Handover Request message, to the bases station system of the destination network;
in step 107, the base station system of the destination network returns a handover response message, for example sending a Handover Request Acknowledge message;
in step 108, after receiving the handover response, the destination network control entity sends a handover response message, for example a CS to PS Handover Response message, to the MSC;
in step 109, the MSC sends a handover command, for example, a Handover Command message, to the base station system of the original network;
in step 110, if the UE-1 also has data services, the destination network control entity will also send a handover response, for example a Forward Relocation Response message, to a control entity of the data domain of the original network;
in step 111, after step 110, the original network data domain control entity sends a handover command, for example a Relocation Command message, to the base station system of the original network;
in step 112, the base station system of the original network sends a handover command, for example a HO from UTRAN/GERAN message, to the UE-1;
in step 113, the UE-1 executes handover, and hands over to the destination network, and then sends an attachment request, for example an Attach Request message, to the destination network control entity;
in step 114, the destination network control entity sends an attachment agreement message, for example an Attach Accept message, to the UE-1.